Four projects are described in this application for a competing renewal of a Research Scientist Award. The first project is a study of the behavioral economics of polydrug abuse: rhesus moneys serve as subjects. Two reinforcing drugs are made concurrently available during daily 3-hr sessions with each drug in a separate reservoir. The objective is to determine how increases in the price of one drug alter intake of both the drugs. Price is determined by the size of the fixed-ratio schedule for the primary drug. The price of the second drug is held constant. If consumption of the second drug increases with increases in price of the primary drug, then the second drug functions as an economic substitute. If consumption of the second drug remains constant then the second drug is an independent commodity, and if consumption of the second drug decreases, then the second drug is an economic complement. The second project is a conceptually similar study that is being conducted in parallel with human subjects. The third project also concerns polydrug abuse, and the design of these studies involves placing a combination of two drugs in a single reservoir, and comparing the relative reinforcing effects of the combination to different concentrations of the constituent. These studies will permit the determination of whether the combination is infra-additive, additive, or supra-additive relative to its constituents. Each of these three projects represents an R01 grant that has been funded. The fourth project concerns studies with rats where the rats' lever pressing is reinforced by experimenter-administered response-contingent injections of drugs of abuse. The injections are delivered SQ. This procedure with rats permits long-term studies that can extend over much of the rats' life span. The studies with rats share with the oral self -administration studies in monkeys the common feature of an experimental preparation that permits studies over long periods of time.